In general, a hydraulic system for a construction machine, particularly, an excavator includes an engine which generates power, a main hydraulic pump which discharges hydraulic oil by being operated by power transmitted from the engine, a plurality of actuators which performs work, an operating unit which is manipulated to operate a desired actuator of a working machine, and a main control valve which distributes required hydraulic oil to a designated actuator by the operation of the operating unit.
However, unlike an excavator, the construction machine, particularly, a loader is a construction machine used to load excavated soil and the like onto a transport vehicle, and typically, a wheel type loader, which has wheels, is widely used because the loader needs to repeatedly perform an operation of collecting and putting soil into a bucket thereof and an operation of loading the soil onto the transport vehicle. In general, the wheel type loader having wheels is called a wheel loader.
In general, a system, which uses an engine and an electric motor as a common power source and has an electricity storage unit which is an electrical energy storage device, is referred to as a hybrid system. An automobile, which adopts the hybrid system, is called a hybrid automobile, and construction heavy equipment such as an excavator, which adopts the hybrid system, is called a hybrid construction machine.
Recently, to cope with a rapid increase in oil prices and meet various types of regulations concerning the environment, researches are being actively conducted on a hybrid construction machine that improves fuel economy by storing surplus power from the engine into the electricity storage unit and supplementing insufficient power of the engine from the electricity storage unit.
FIG. 1 is a graph related to a traction load of a wheel loader driving device in the related art, and FIG. 2 is a graph for explaining a capacity of an electric motor of the wheel loader driving device.
As illustrated in FIG. 1, the traction load of the wheel loader driving device in the related art requires high torque at a low speed (RPM), and requires low torque at a high speed. That is, as illustrated in FIG. 2, in order to implement the wheel loader driving device as a hybrid type, a high-capacity electric motor and an inverter are required to satisfy the traction load associated with a capacity of the electric motor. As a result, there is a problem in that because of the nature of the traction load of the wheel loader, it is difficult to develop a hybrid type wheel loader driving device.